Fuel ethanol is currently made from feedstocks such as corn starch, sugar cane, and sugar beets. The production of ethanol from these sources cannot grow much further, as most of the farmland suitable for the production of these crops is in use. In addition, these feedstocks can be costly since they compete with the human and animal food chain. Finally, the use of fossil fuels, with the associated release of carbon dioxide and other products, for the conversion process is a negative environmental impact of the use of these feedstocks.
The production of fuel ethanol from cellulosic feedstocks provides an attractive alternative to the fuel ethanol feedstocks used to date. Cellulose is the most abundant natural polymer, so there is an enormous untapped potential for its use as a source of ethanol. Cellulosic feedstocks are also inexpensive, as they do not have many other uses. Another advantage of producing ethanol from cellulosic feedstocks is that lignin, which is a byproduct of the cellulose conversion process, can be used as a fuel to power the conversion process, thereby avoiding the use of fossil fuels. Several studies have concluded that, when the entire cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The cellulosic feedstocks that are the most promising for ethanol production include (1) agricultural wastes such as corn stover, wheat straw, barley straw, canola straw, rice straw, and soybean stover; (2) grasses such as switch grass, miscanthus, cord grass, and reed canary grass, (3) forestry wastes such as aspen wood and sawdust, and (4) sugar processing residues such as bagasse and beet pulp.
Regardless of the feedstock used, the first step involves handling and size reduction of the material. The feedstock must be conveyed into the plant. This is contemplated to be carried out by trucks, followed by placing the feedstock on conveyor belts to be conveyed into the plant. The feedstock particles must then be reduced to the desired size to be suitable for handling in the subsequent processing steps.
The first process step is a chemical treatment, which usually involves the use of steam along with acid or alkali to break down the fibrous material. The chemical treatment is carried out for either of two primary processes—acid hydrolysis and enzymatic hydrolysis—used to convert the feedstock to sugar.
In the acid hydrolysis process, the feedstock is subjected to steam and sulfuric acid at a temperature, acid concentration, and length of time that are sufficient to hydrolyze the cellulose to glucose and hemicellulose to xylose and arabinose. The sulfuric acid can be concentrated (25-90% w/w) or dilute (3-8% w/w). The glucose, xylose and arabinose are then fermented to ethanol using yeast, and the ethanol is recovered and purified by distillation. A problem with concentrated acid hydrolysis is that the high levels of concentrated acid required necessitate the recovery and re-use of over 99% of the acid in the process. The recovery of this high proportion of acid is especially difficult due to the high viscosity and corrosivity of concentrated acid.
In the enzymatic hydrolysis process, the steam temperature, concentration of acid, and treatment time are chosen to be significantly milder than that in the acid hydrolysis process, such that the exposed cellulose surface area is greatly increased as the fibrous feedstock is converted to a muddy texture. Much of the hemicellulose is hydrolyzed, but there is little conversion of the cellulose to glucose. The cellulose is hydrolyzed to glucose in a subsequent step that uses cellulase enzymes, and the steam/acid treatment in this case is known as pretreatment. The acids used in pretreatment include sulfuric acid in steam explosion and batch and continuous flow pretreatments and also sulfurous acid and phosphoric acid.
The hydrolysis of the cellulose, whether by acid hydrolysis or by cellulase enzymes after pretreatment, is followed by the fermentation of the sugar to ethanol. The ethanol is then recovered by distillation.
There are several problems that must be overcome in order for the conversion of cellulosic biomass to sugar or ethanol to be commercially viable. In particular, there is a large amount of inorganic salt present in the feedstock and generated in the process. The inorganic salt has an adverse impact on the pretreatment, enzymatic hydrolysis, and yeast fermentation processes. In addition, the purchase of the acid and the alkali and the disposal of the salt is costly.
Neutral salts consist of cation(s), which provide a positive charge, and anion(s), which provide a negative charge. The most prevalent element in the feedstock that is a source of cations is potassium. Other elements in the feedstocks that are significant sources of cations include calcium, sodium, and magnesium, at concentrations of about ⅓, 1/7, and 1/10 that of potassium. Most of the potassium, calcium, sodium, and magnesium in the feedstocks is complexed with organic compounds, such as proteins or carboxylic acids, or exists in the form of oxides or oxalates. The feedstocks are slightly alkaline with this “excess” of cations, as the concentration of anions is low.
The addition of, for example, sulfuric acid to the feedstock as part of the chemical treatment forms the mixtures of sulfuric acid and acidified sulfate salts, which include potassium bisulfate, calcium sulfate, sodium bisulfate, and magnesium bisulfate. Analogous acidified salts are formed with the use of other acids. These acidified salts are more water soluble than the complexed cations, and are released into solution upon acid addition. The presence of the cations thereby increases the amount of acid required in the chemical treatment. The high acid and acidic salt concentrations result in some degradation of sugar, including xylose, during the pretreatment.
The acid used in the pretreatment must be neutralized prior to the enzymatic hydrolysis of the cellulose or fermentation of the sugar. Cellulase enzymes produced by the fungus Trichoderma, which are the leading sources of cellulase for cellulose conversion, exhibit optimum activity at pH 4.5 to 5.0. These enzymes exhibit little activity below pH 3. Microbes that ferment the sugar include yeast and Zymomonas bacteria. The yeast are active at pH 4-5 while the Zymomonas are active at pH 5-6. An acidic chemical treatment is often carried out at a pH of about 0.8 to 2.0, so a significant amount of alkali must be added to increase the pH to the range that is required for microbial fermentation and enzymatic hydrolysis.
When an acidic pretreatment is carried out, the alkaline that is usually used for neutralization of the acid is sodium hydroxide, but potassium hydroxide and ammonium hydroxide have also been reported. The high levels of these compounds that are required increase the cost of the process.
Although the neutralized slurry is at a pH range that is compatible with yeast or fermenting bacteria or cellulase enzymes, the inorganic salt concentration is high enough to be inhibitory to the microbes or enzymes. The inorganic salt can also cause a degradation of the sugar, particularly the xylose, in evaporation and distillation processes that are carried out downstream of the hydrolysis.
Alkali that is used during processing of the lignocellulosic feedstock can be either soluble or insoluble. An example of an insoluble alkali is lime, which is used to precipitate inhibitors of cellulase enzymes arising from the pretreatment. This process is known as over-liming and involves adding lime to the pretreated feedstock until an alkaline pH of between 9 and 12 is achieved. The limed material is then adjusted to pH 5 prior to enzyme hydrolysis using phosphoric acid, carbon dioxide or other convenient acids. Since overliming precipitates some of the inhibitors of the cellulase enzymes, it results in an improved enzymatic hydrolysis of the cellulose with existing pretreatment technologies. However, there are numerous problems associated with overliming including (1) disposal of the lime; (2) calcium precipitation which leads to downstream scaling; (3) the expense of the lime; and (4) the fact that the treatment is not completely effective in removing inhibitors of enzymes and yeast.
Wooley et al. (In Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzyme Hydrolysis Current and Future Scenarios, (1999) Technical Report, National Renewable Energy Laboratory pp. 16-17) describe a process of treating lignocellulosic material utilizing over-liming following acid pretreatment. Milled wood chips are first pretreated with dilute sulfuric acid followed by enzyme hydrolysis and fermentation. Following pretreatment, the resulting liquid and solids are flash cooled to vapourize a large amount of water and inhibitors of the downstream fermentation reaction. After ion exchange to remove acetic acid, the material is over-limed by adding lime to raise the pH to 10. The liquid is then adjusted to pH 4.5 which results in the formation of gypsum crystals (CaSO4). These crystals can be removed from the liquid by hydrocyclone and rotary drum filtration in series. Although the process describes the removal of gypsum after acid treatment, the investigators do not address the problems associated with removal of insoluble calcium salt.
U.S. Pat. No. 6,043,392 (Holtzapple et al.) employs a pretreatment step with lime prior to recovering volatile fatty acids produced during the fermentation of lignocellulosic biomass by anaerobic or thermophilic bacteria. After the treatment, lime is removed by draining the lime-containing water from the biomass, followed by fermentation with anaerobic bacteria. The anaerobic organisms then convert the biomass to organic acids such as acetic acid, proprionic and butyric acids. The organic acids produced by these fermentation processes can be concentrated and converted to ketones by pyrolysis in a thermal converter. Any soluble and insoluble minerals present after pyrolysis can be recovered and sold as a fertilizer. Alternatively, the organic acids are treated with a tertiary amine and carbon dioxide to produce an acid/amine complex that decomposes to form an acid and an amine with different volatilities. The acid can then be separated from the amine by distillation and precipitated minerals that accumulate in the bottoms of the distillation column can be recovered. Although Holtzapple et al. describe an effective method for the isolation of organic acids produced during fermentation using an alkaline pretreatment, they do not demonstrate the applicability of their process using an acidic pretreatment, nor did they demonstrate the effectiveness of their pretreatment in improving enzymatic hydrolysis.
U.S. Pat. No. 6,478,965 (Holtzapple et al.) discloses a method for isolating carboxylate salts formed as a product during the fermentation of lignocellulosic biomass by anaerobic bacteria. A fermentation broth, which contains dilute carboxylate salt in aqueous solution, is contacted with a low molecular weight secondary or tertiary amine which has a high affinity for water and a low affinity for the carboxylate salt. This allows the water to be selectively extracted while the carboxylate salt remains in the fermentation broth and becomes concentrated so that it can be easily recovered. The carboxylate salt may be further concentrated by evaporation, dried or converted to a more concentrated carboxylic acid solution. While Holtzapple et al. describe an effective method for the isolation of carboxylate fermentation products, they do not address the recovery of inorganic salts from the feedstock itself or inorganic salts arising from the acids and bases used during the processing of the lignocellulosic feedstocks. Furthermore, the process disclosed does not include a step of acidic pretreatment prior to enzymatic hydrolysis and fermentation.
U.S. Pat. No. 5,124,004 (Grethlein et al.) discloses a method for concentrating an ethanol solution by distillation. The method first involves partially concentrating the ethanol solution by distillation and withdrawing a vapour stream. Next, the condensation temperature of the vapour is raised above the evaporation temperature of a re-boiler liquid used in the process (a heat-sink liquid). The vapour stream is then used to heat the re-boiler liquid and partially enriched vapour is then removed and condensed. The condensed stream is introduced to an extractive distillation column and concentrated in the presence of an added salt to increase the volatility of the ethanol. The invention provides the benefit that the heat requirement of distillation is reduced since vapour required to heat the system does not need to be provided by an external source. However, there is no discussion of recovering and removing the salts added during the final distillation step.
U.S. Pat. No. 5,177,008 (Kampen) discloses the recovery of fermentation by-products, namely glycerol, betaine, L-pyroglutamic acid, succinic acid, lactic acid and potassium sulfate, produced during the manufacture of ethanol from sugar beets. The process involves fermenting the raw material, collecting the ethanol by distillation and then recovering the by-products in the remaining still bottoms. The by-products are isolated by first centrifuging the still bottoms and performing microfiltration to further clarify the solution. The resulting permeate is then concentrated to a solids concentration of 50-75%. The concentrated solution is first subjected to a crystallization step to recover potassium sulfate and then passed to a chromatographic separation step for the subsequent recovery of glycerol, betaine, succinic acid, L-pyroglutamic acid or lactic acid. The potassium sulfate is present in the raw material and its concentration is increased by cooling the solution and/or by the addition of sulfuric acid as part of the crystallization. The process of Kampen has several advantages such as energy and water savings and high solids concentrations. However, there is no discussion of a chemical pretreatment of lignocellulosic material with acid or an alkali neutralization step prior to enzymatic hydrolysis and fermentation and the associated problems with the presence of sodium and magnesium salts arising from such a pretreatment. Furthermore, since Kampen et al. used sugar beets, they were able to crystallize potassium sulfate directly from the still bottoms, and they do not address the recovery from still bottoms of salt mixtures with high levels of impurities that do not crystallize. Acid pretreatment of lignocellulosic feedstocks results in mixtures of inorganic salts in the still bottoms that cannot be directly crystallized.
U.S. Pat. Nos. 5,620,877 and 5,782,982 (Farone et al.) disclose a method for producing sugars from rice straw using concentrated acid hydrolysis which, as set out above, is not a preferred pretreatment method. The method results in the production of quantitative yields of potassium silicate. In this method, the rice straw is treated with concentrated sulfuric acid at a concentration of between 25% and 90%. The resulting mixture is then heated to a temperature to effect acid hydrolysis of the rice straw. Subsequently, the mixture is separated from the remaining solids by pressing. The pressed solids can then be treated with 5% to 10% sodium hydroxide to extract silicic acid. Following the treatment with sodium hydroxide, the solids are heated and then pressed and washed with water to extract a liquid. The extracted liquid is then treated with an acid, which results in the formation of a precipitate that can be separated by filtration. The filtered material is then treated with bleach to produce silica gel that can be further treated to produce sodium silicate, potassium silicate or other useful materials. The method also employs a neutralization step using lime to precipitate soluble inorganic salts present in a sugar stream produced during fermentation. Lime is an insoluble base that can build up on process equipment downstream of its point of addition and decrease the efficiency of the process.
WO 02/070753 (Griffin et al.) discloses a leaching process to remove alkali from lignocellulosic feedstocks thereby decreasing the acid requirement for chemical treatment. The process includes milling the feedstock, followed by preconditioning it with steam and then contacting the feedstock with water to leach out the salts, protein, and other impurities. The water containing these soluble compounds is then removed from the feedstock. This process decreases the acid requirements in the subsequent pretreatment process, which increases the yield of xylose after pretreatment. However, the costs and problems associated with the salt arising from the acid or alkali added for chemical treatment and the alkali or acid added after chemical treatment for adjustment of the pH are not addressed. Furthermore, disposing of the leachate is inconvenient and adds to the cost of the process.
U.S. Pat. No. 4,321,360 (Blount) discloses the preparation of ethanol from lignocellulosic feedstocks; however, there is no discussion of salt processing or recovery arising during this process. U.S. Pat. No. 6,608,184 (Blount) describes the production of ethanol, salt, and several other organic products from sewer sludge comprising sewered cellulose waste material (rather than a lignin-cellulose material). This process involves mixing sewer sludge with water and sodium hydroxide, or an acid (sulfuric or hydrochloric acid). The slurry containing acid or alkali is then heated to hydrolyze the cellulose in the sludge, and an excess of water is added to dissolve the organic compounds. The aqueous material is then separated from the insolubles and evaporated to concentrate the solution and crystallize out the carbohydrates. The carbohydrates are filtered off, slurried in water, and fermented to ethanol using yeast. The aqueous solution containing ammonium sulfate and other compounds may then be used as a fertilizer. Alternatively, the salt is separated from the sugar by membrane filtration and then the salt is evaporated and dried.
U.S. Pat. No. 6,709,527 (Fechter et al.) discloses a process of treating an impure cane-derived sugar juice to produce white sugar and white strap molasses. The process involves subjecting the sugar juice to microfiltration/ultrafiltration to decrease the levels of suspended solids, organic non-sugar impurities and/or colour. The sugar juice is next subjected to ion exchange with a strong acid cation exchange resin in the hydrogen form and then to ion exchange with an anion ion exchange resin in the hydroxide form. Potassium-based fertilizer components can be obtained by regenerating the strong acid cation exchange resin with a strong acid such as hydrochloric acid or nitric acid to produce an acid stream rich in potassium salt. Ammonium-based fertilizer components can be obtained by regenerating the anion ion exchange resin with a strong or weak base such as sodium or potassium hydroxide and ammonium hydroxide to obtain an alkaline stream which is rich in nitrogen. After ion exchange, the resulting sugar solution is concentrated to produce a syrup, which is crystallized twice to produce impure crystallized sugar and white strap molasses. Although the patent describes the production of potassium and ammonium-based fertilizer components, there is no discussion of the recovery of inorganic salts arising from an acidic pretreatment and neutralization step.
U.S. Pat. No. 4,101,338 (Rapaport et al) disclose the separation of sucrose from impurities in sugar cane molasses. Rapaport et al. teach the pretreatment of a molasses stream to remove a significant amount of organic non-carbohydrate impurities and colour. The pretreatment can be carried out by precipitation with iron salts, such as ferric chloride or ferric sulfate. The insoluble flocculants are then removed from the molasses stream and the soluble iron salts are removed by the addition of lime and phosphoric acid or phosphate salts. The pretreatment may also be carried out by other processes which include: centrifugation, with removal of the cake; precipitation by adding ethanol to the molasses stream; and filtering the molasses across a membrane of cellulose acetate. Regardless of the pretreatment process, the purpose is to decrease the amount of organic non-carbohydrate impurities so that a subsequent step of ion exclusion chromatography will separate the carbohydrate fraction from the dissolved impurities. Rapaport et al. report that the pretreatment decreased the ash content to 10% and the organic non-sugar content to 16.3% of the solids present.
Organic non-carbohydrate impurities, within a lignocellulosic system, cannot be removed by the methods of U.S. Pat. No. 4,101,338 (Rapaport et al.) According to Rapaport's method, the amount of solids precipitated by iron salts or ethanol is modest and no solids are removed by centrifugation. By contrast, the sugar streams produced during the processing of lignocellulosic feedstock have a much higher level of organic non-carbohydrate impurities and inorganic salts. Rapaport et al. do not address the processing of such concentrated streams. Furthermore, the use of cellulose acetate membranes in a lignocellulosic system would not be feasible since such membranes would be destroyed by cellulase enzymes.
A process for the pretreatment, enzymatic hydrolysis and sugar fermentation of lignocellulosic feedstocks is required that addresses the problems associated with high inorganic salt concentrations in the feedstock and in the process. The development of such a process would represent a significant step forward in the commercialization of ethanol production from lignocellulosic biomass.